Powder coatings are a well known type of thermosetting coating and carboxyl group bearing polyesters are used to formulate a particular type of powder coating. In particular polyesters with a substantial content of carboxylic acid groups are typically formulated with epoxide compounds to yield powders which can be applied to various substrates by electrostatic spray or fluidized bed and then cured by baking. The cure or crosslinking to a thermoset solvent resistant state is the result of the reaction between the carboxyl groups and the epoxy groups.
The art has found it to be a practical necessity to include a curing catalyst in such polyester/epoxy compound compositions. The uncatalyzed rate of reaction has been found to be too slow to suit the baking schedules established in the industry. The alternatives available to the end user have forced the suppliers of polyester-epoxy based powder coatings to provide products which cure at rates competitive to these alternatives.
However, a difficulty has arisen in achieving a uniform distribution of the curing catalyst in the powder coating composition. The polyester component is typically manufactured by one supplier and the epoxy component is manufactured by another, although some manufacturers offer both components. In any case the powder paint formulator typically combines these ingredients and others, such as pigment and flow aids, in an extruder. The extruder is fairly efficient in achieving a uniform mixture of these ingredients. But the curing catalyst is typically employed in amounts of less than one weight percent, often less than 0.5 weight percent and it is difficult to obtain a good distribution of such a small amount of an ingredient in the typical compounding extruder, particularly when the catalyst is a liquid at the compounding temperature. At such low levels the liquid catalyst is unable to uniformly wet the particles of solid material.
A non-homogeneous distribution of cure catalyst may lead to localized high and low catalyst concentration regions even within a single coating batch thus giving inconsistent quality. High catalyst concentration can cause a premature curing reaction between the epoxy groups and the carboxyl groups during the extrusion compounding and thus result in gel formation. These gels can then cause defects in the finished coating. Over-catalyzed material is also subject to gel formation during extended storage of the powder coating composition thus undesirably limiting its shelf life. On the other hand, under-catalyzed material may fail to achieve an adequate state of cure to display the required properties under a given set of baking conditions.
The common compounding of uniformly incorporating a low level additive by preparing a master batch poses special difficulties for these curing catalysts. Because these catalysts have a tendency to chemically interact with either of the chemically active components (the carboxyl polyester and the epoxy compound) there is concern about degradation of the master batch with its high concentration of catalyst either during its preparation or its incorporation into the powder coating composition. This approach involves subjecting the high catalyst content master batch composition to two heat histories; the first one in preparing the master batch by combining a major ingredient with a high level of catalyst and the second one in diluting the catalyst concentration down to end use levels by compounding the master batch into the mixture of the remainder of the powder coating composition components.
There are also difficulties with adding the curing catalyst to a liquid melt of one of the major components. Incorporation into the epoxy component does not represent a realistic alternative to a supplier of the polyester component who does not also supply an epoxy component. Furthermore, a substantial portion of the market for carboxyl polyester powder coating components is in formulations involving a fairly low content of epoxy component. For instance there is a sub-market for carboxyl polyesters which are compounded with triglycidyl isocyanurate (TGIC), which has a fairly low epoxy equivalent weight. Such formulations involve polyester to TGIC weight blend ratios of around 14:1. Thus incorporating the catalyst into the TGIC would require high concentrations of catalyst raising similar problems to those involved with master batching.
The incorporation of the curing catalyst into a liquid melt of the carboxyl polyester component may cause discoloration of the polyester. At the molecular weights at which these polyesters find utility in powder coatings liquid melts are only achieved at temperatures in the range of 200.degree. C. Furthermore, the common practice in the production of such resins is to transfer the liquid melt directly from the reaction vessel to a flaking belt on which it solidifies to form solid flakes. For a commercial size production vessel this process often takes five or more hours. Thus if a curing catalyst is to be incorporated into a liquid melt of the polyester it must be stable in the melt at temperatures in excess of 200.degree. C. for times in excess of five hours. Unfortunately, a particularly interesting class of curing catalysts, the imidazoles, lacks optimum stability under these conditions and causes discoloration of the polyester.
Although some degree of discoloration can be tolerated in some applications it is undesirable in all cases. In some applications the powder coating composition is so highly pigmented that any discoloration of the composition would be effectively masked. However, there are other applications such as transparent coats in which it is unacceptable. Furthermore powder coating formulators have an aversion to purchasing a discolored raw material even if the discoloration can be masked.
It has now been found that a particular imidazole, 1-methyl imidazole, will display the requisite stability if the carboxyl terminated polyester is prepared in a special manner. A convenient technique for preparing such polyesters is to first prepare a hydroxyl terminated polyester and then react it with a carboxylic acid anhydride to achieve acid termination. Such an approach is taught in U.S. Pat. No. 4,147,737 to Sein et al. The 1-methyl imidazole will cause discoloration if added to a liquid melt of a carboxyl polyester prepared in such a manner. It is necessary to utilize carboxyl polyesters in whose synthesis such anhydrides did not play a significant role.